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S. Schramm

Publications and source records attributed to S. Schramm.

105 records · Page 6Linked to original sources
arXiv1998

Nuclei in a chiral SU(3) model

Nuclei can be described satisfactorily in a nonlinear chiral SU(3)-framework, even with standard potentials of the linear $σ$-model. The condensate value of the strange scalar meson is found to be important for the properties of nuclei even without adding hyperons. By neglecting terms which couple the strange to the nonstrange condensate one can reduce the model to a Walecka model structure embedded in SU(3). We discuss inherent problems with chiral SU(3) models regarding hyperon optical potentials.

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arXiv1998

Electron Propagation in the Field of Colliding Nuclei at Ultrarelativistic Energies

We calculate the asymptotic high-energy amplitude for electrons scattering at one ion as well as at two colliding ions, respectively, by means of perturbation theory. We show that the interaction with one ion eikonalizes and that the interaction with two ions causally decouples. We are able to put previous results on perturbative grounds and propose further applications for the obtained rules for interactions on the light cone. The formalism will be of use for the calculation of Coulomb corrections to electron-positron pair creation in heavy ion collisions. Finally we discuss the results and inherent dangers of the employed approximations.

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arXiv1997

Temperature dependence of instantons in QCD

We investigate the temperature dependence of the instanton contents of gluon fields, using unquenched lattice QCD and the cooling method. The instanton size parameter deduced from the correlation function decreases from 0.44fm below the phase-transition temperature $T_c$ ($\approx 150$MeV) to 0.33fm at 1.3 $T_c$. The instanton charge distribution is Poissonian above $T_c$, but it deviates from the convoluted Poisson at low temperature. The topological susceptibility decreases rapidly below $T_c$, showing the apparent restoration of the $U(1)_A$ symmetry already at $T \approx T_c$.

hep-lat↗
arXiv1997

Chiral Lagrangian for strange hadronic matter

A generalized Lagrangian for the description of hadronic matter based on the linear $SU(3)_L \times SU(3)_R$ $σ$-model is proposed. Besides the baryon octet, the spin-0 and spin-1 nonets, a gluon condensate associated with broken scale invariance is incorporated. The observed values for the vacuum masses of the baryons and mesons are reproduced. In mean-field approximation, vector and scalar interactions yield a saturating nuclear equation of state. We discuss the difficulties and possibilities to construct a chiral invariant baryon-meson interaction that leads to a realistic equation of state. It is found that a coupling of the strange condensate to nucleons is needed to describe the hyperon potentials correctly. The effective baryon masses and the appearance of an abnormal phase of nearly massless nucleons at high densities are examined. A nonlinear realization of chiral symmetry is considered, to retain a Yukawa-type baryon-meson interaction and to establish a connection to the Walecka-model.

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arXiv1997

Pion and Rho Structure Functions from Lattice QCD

We calculate the lower moments of the deep-inelastic structure functions of the pion and the rho meson on the lattice. Of particular interest to us are the spin-dependent structure functions of the rho. The calculations are done with Wilson fermions and for three values of the quark mass, so that we can perform an extrapolation to the chiral limit.

hep-lat↗
arXiv1997

Structure of the Vacuum in Nuclear Matter - A Nonperturbative Approach

We compute the vacuum polarisation correction to the binding energy of nuclear matter in the Walecka model using a nonperturbative approach. We first study such a contribution as arising from a ground state structure with baryon-antibaryon condensates. This yields the same results as obtained through the relativistic Hartree approximation of summing tadpole diagrams for the baryon propagator. Such a vacuum is then generalized to include quantum effects from meson fields through scalar-meson condensates. The method is applied to study properties of nuclear matter and leads to a softer equation of state giving a lower value of the incompressibility than would be reached without quantum effects. The density dependent effective sigma mass is also calculated including such vacuum polarisation effects.

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arXiv1996

Delta Decay in the Nuclear Medium

The $Δ$ decay in the nuclear medium is calculated in the relativistic meson-nucleon model. The delta spreading width is calculated and compared with the Pauli-blocked $π$N decay width. The influence of relativistic mean fields is also studied. We stress the importance of understanding the delta spreading width in interpreting experiments involving delta resonances.

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arXiv1996

Phase Transition in the chiral $σ$-$ω$ model with dilatons

We investigate the properties of different modifications to the linear $σ$-model (including a dilaton field associated with broken scale invariance) at finite baryon density $ρ$ and nonzero temperature $T$. The explicit breaking of chiral symmetry and the way the vector meson mass is generated are significant for the appearance of a phase of nearly vanishing nucleon mass besides the solution describing normal nuclear matter. The elimination of the abnormal solution prohibits the onset of a chiral phase transition but allows to lower the compressibility to a reasonable range. The repulsive contributions from the vector mesons are responsible for the wide range of stability of the normal phase in the $(μ,T)$-plane. The abnormal solution becomes not only energetically preferable to the normal state at high temperature or density, but also mechanically stable due to the inclusion of dilatons.

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arXiv1996

Detection of Atmospheric Neutrinos and Relativistic Nuclear Structure Effects

Neutrino-nucleus cross sections for the detection of atmospheric neutrinos are calculated in relativistic impulse and random phase approximations. Pion production is estimated via inclusive Delta-hole nuclear excitations. The number of pion events can confuse the identification of the neutrino flavor. As a further source of pions we calculate coherent pion production (where the nucleus remains in the ground state). We examine how these nuclear structure effects influence atmospheric neutrino experiments and study possible improvements in the present detector simulations.

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arXiv1995

Delta Excitations in Neutrino-Nucleus Scattering

We derive the contribution of $Δ$-h excitations to quasielastic charged-current neutrino-nucleus scattering in the framework of relativistic mean-field theory. We discuss the effect of $Δ$ production on the determination of the axial mass $M_A$ in neutrino scattering experiments.

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arXiv1994

Instanton content of finite temperature QCD matter

We investigate the temperature dependence of the instanton content of gluon fields and their contribution to quark correlation using quenched lattice QCD and the cooling method. We found a suppression of the topological susceptibility at finite temperature, agreeing with the PCAC expectation at low temperature and enhanced suppression for temperatures at and above the deconfinement transition. For temperatures up to about 334 MeV, the topological charge correlation agrees well with a single instanton profile, though the size parameter seems to change across the phase transition. The screening wave functions for the pion and the rho become slightly more compact at higher temperatures. Lattice cooling shows no contribution from the instantons to the screening wave functions at temperatures above the phase transition.

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arXiv1993

Can Plasma Scattering Mimic a Cosmological Red-Shift?

We investigate the extent to which the scattering of light from plasma fluctuations can mimic a cosmological red-shift, as has been suggested by Wolf et al. A plasma model for the plasma structure function results in a spectrum of scattered light very different from that associated with a constant red-shift, implying that the "Wolf effect" cannot be involved to perturb the cosmological distance scale.

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